Method and apparatus for aligning and setting the axis of rotation of spindles of a multi-body system

ABSTRACT

A method and apparatus is disclosed for polishing a semiconductor wafer. A polishing pad including a first surface and a semiconductor wafer including a second surface are aligned to each other. To allow alignment of an axis of rotation of the surfaces, at least one of the first and second surfaces includes an adjustable axis of rotation. After the axis of rotation of the first and second surfaces is aligned, the adjustable axis of rotation is set, preferably with a magneto-rheological fluid or similarly acting material, to maintain a fixed position. Thereafter, the polishing pad is utilized to polish the semiconductor wafer.

FIELD OF THE INVENTION

[0001] The invention relates to the alignment of the axis of multi-bodysystems, including but not limited to multi-body systems for thepolishing of semiconductor wafer surfaces. More specifically, thepresent invention relates to a method and system for dynamicself-alignment of the axis of rotation of a semiconductor wafer surfaceand a polishing surface.

BACKGROUND

[0002] Semiconductor wafers are commonly constructed in layers, where aportion of a circuit is created on a first level and conductive vias aremade to connect up to the next level of the circuit. After each layer ofthe circuit is etched on the wafer, an oxide layer is put down allowingthe vias to pass through but covering the rest of the previous circuitlevel. Each layer of the circuit can create or add unevenness to thewafer that must be smoothed out before generating the next circuitlayer.

[0003] Chemical mechanical polishing (CMP) techniques are used to polishand planarize the raw wafer and each layer of circuitry added. AvailableCMP systems, commonly called wafer polishers, often use a rotating wafercarrier head that brings the wafer into contact with a polishing padrotating in the plane of the wafer surface to be planarized. A chemicalpolishing agent or slurry containing microabrasives is applied to thepolishing pad to polish the wafer. The wafer carrier head then pressesthe wafer against the rotating polishing pad and is rotated to polishand planarize the wafer. The mechanical force for polishing is derivedfrom the rotating table speed and the downward force on the wafercarrier head.

[0004] A conventional way to axially align the rotating polishing padwith the wafer carrier is to use a rigid surface on one body, such as abody that supports the polishing pad, and a gimbal on the other. Aproblem occurs when the polishing surface of the rotating polishing padis smaller than the wafer surface, and especially as the polishingsurface moves off an edge of the wafer during polishing. Since thegimbaled surface tends to tilt at the edge, alignment of the wafer tothe polishing surface becomes difficult, and is preferably accomplishedby using all rigid surfaces. Many polishing techniques, such as CMP,however, require at least one dynamic self-aligning surface to align thepolished layer to a previously processed underlying layer of the wafer.

[0005] Thus, there is a need for a method for polishing wafers where thewafer is dynamically aligned to a polishing surface and then rigidlyheld in place when the alignment is accomplished.

BRIEF SUMMARY

[0006] A method and system are disclosed for automatically aligning andsetting an axis of rotation of a semiconductor wafer to a polishing pad,for example, without using a gimbal mechanism usually incorporated intoa wafer head, i.e., wafer carrier. After an angle of the axis ofrotation is aligned to the pad, the angle is fixed in place. In thismanner, the polishing pad can effectively polish a semiconductor waferthat is attached to the wafer head.

[0007] According to an aspect of the invention, a polishing pad includesa first surface and a semiconductor wafer includes a second surface. Topolish the semiconductor wafer, axis of rotation of the first and secondsurfaces are aligned to each other. To allow axial alignment of thesurfaces, at least one of the first and second surfaces includes anadjustable axis of rotation. After the axis of rotation of the first andsecond surfaces is aligned, the adjustable axis of rotation is set tomaintain the adjusted position. Thereafter, the polishing pad polishesthe semiconductor wafer, for example, in a radially symmetric fashion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 illustrates a side view of a prior art chemical mechanicalpolishing system.

[0009]FIG. 2 illustrates a side view of a polishing system according toa first embodiment.

[0010]FIG. 3 illustrates a side view of a polishing system according toa second embodiment.

[0011]FIG. 4 is a flow chart representation of a polishing methodaccording to the preferred embodiments.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0012] Referring to the drawings, and particularly FIG. 1, aconventional angular-motion system is shown for polishing asemiconductor wafer. The system includes at least one wafer carrier head10 that includes wafer carrier surfaces 12. Also included in the systemis a polish head 14 that includes a polishing surface 16. As shown, thesystem is a three-body system, i.e., one polish head 14 and two wafercarrier heads 10. Of course other combinations of polish heads 14 towafer carrier heads 10 could be used, such as one polish head 14 and onewafer carrier head 10.

[0013] To rotate the polish head 14 and the wafer carrier heads 10, thesystem includes spindle shafts 18 connected with each of the heads 10,14. As the spindle shafts 18 rotate, the polishing surface 16 and thewafer carrier surfaces 12 rotate with respect to each other. Therotating wafer carrier head 10 brings the wafer into contact with apolishing pad located on the polishing surface 16. The polishing padrotates in the plane of the wafer surface to be polished. The chemicalpolishing agent or slurry is applied to the polishing pad to polish thewafer. In other embodiments, a fixed abrasive polishing pad may be used.The wafer carrier head 10 then presses the wafer against the rotatingpolishing pad and is rotated to polish the wafer.

[0014] To align a surface of the semiconductor wafer to the polishingsurface 16 during the polishing, the wafer carrier heads 10 typicallyinclude gimbals 20. Because of their flexible nature, the gimbals 20accommodate changes in parallelism between the wafer carrier heads 10and the polishing surface 16. In this manner, a difference in an angleθ1 between the axis of the polish head 14 spindle shaft 18 and the wafercarrier head 10 spindle shaft 18 can be accommodated. A problem occurs,however, in that the gimbaled wafer carrier head 10 can become unalignedwith the polish head 14 during polishing, especially as the polish head14 moves of an edge 22 of the wafer carrier head 10.

[0015] Referring to FIG. 2, a system is shown according to the preferredembodiments for polishing a semiconductor wafer 24. Those skilled in theart will appreciate that while a two-body system is shown, the belowdescribed system also applies to multi-body systems. A polishing pad 26includes a polishing surface 28. The semiconductor wafer 24, which maybe comprised of silicon scaled to the dimensions of a given circuit,includes a semiconductor surface 30 to be polished that opposes thepolishing surface 28 of the polishing pad 26. The polishing pad 26engages a polishing head 32 and the polishing head 32 connects with apolishing spindle 34. The semiconductor wafer 24 engages a wafer head 36and the wafer head 36 connects with a wafer spindle 38. Thesemiconductor wafer 24 is held in place by a retention device (notshown) and/or by vacuum. At least one of an angle θ2 of axis of rotationof the wafer head 36 and the polishing head 32, and preferably the waferhead 36, is adjustable.

[0016] The adjustable axis of rotation of the wafer head 36, forexample, is adjustable by angle θ2. To adjust the angle θ2, bearings 40connect with the wafer spindle 38 and at least one fin 42, andpreferably multiple fins, attach to the bearings 40. The fins 42 enter acasing 44, for example, via a seal 46. The seal 46 allows the fins 42 tomove relative to the casing 44 while maintaining the fluid 48 in thecasing 44. The fins 42 increase load-bearing area of the wafer spindle38 to reduce load per unit area encountered by the fluid 48. Referringalso to FIG. 3, in an alternate embodiment, the fins 42 are eliminatedwhen the fluid 48 can harden sufficiently to support the wafer spindle38 without the fins 48.

[0017] Preferably, the fluid 48, enclosed in the casing 44, encompassesthe fins 42. Alternatively, the fluid 48 can directly encompass thewafer spindle 38. The fluid 48 is preferably a magneto-rheological fluidthat is commercially available and includes iron particles in the fluid.Those skilled in the art will appreciate that other fluids with similarproperties to the magneto-rheological fluid could be used that performthe same functions of the magneto-rheological fluid as described below.Other materials, such as a gas or a powder, could be used that allowchange in viscosity, so long as the response time of the state change isfast enough for the application. In the present embodiments, a responsetime in the order of milliseconds is preferred.

[0018] In a first state, the magneto-rheological fluid 42 exhibits theproperties of a liquid, and in a second state, when a magnetic field isapplied at or proximate to the magneto-rheological fluid 42, the fluid48 undergoes an apparent change of state and exhibits the properties ofa solid or a fluid with high viscosity, i.e., high resistance to flow.When the fluid 48 is solid, the fluid 48 maintains a position of thefins 46, thereby maintaining an axial position of the wafer spindle 38.

[0019] To apply a magnetic field to the fluid, the system includes atleast one flux guide such as electromagnetic coil 50. In the preferredembodiment, the coils 50 have a substantially circular cross-section.Preferably, the flux guides' shapes and sizes emanate the desired fluxintensity to the desired locations. It should be noted, however, thatflux guides are not limited to the illustrated dimensions, lengths, orthe cross-sections of the coils 50 shown in the accompanying figures.Thus, the substantially circular cross-section shapes of the coils 50,their positions proximate to the casing 44, and their illustrateddiameters, illustrate only a few of the many forms that this aspect ofthe flux guide can take. The coils 50, for example, can have a polygonalcross-section and/or be positioned across the entire or a portion of thewidth or the length of the casing 44. Preferably, the magnetic field canbe tuned to control the viscosity of the fluid to control the rate ofaxial adjustment, depending on the application requirements. Asdescribed in more detail below, the magnetic field can be alternatelyapplied and disengaged to continually adjust and set the axis ofrotation of the wafer spindle.

[0020] Referring to FIG. 4, a method for using the above-describedsystem is disclosed. To polish the semiconductor wafer 24, thesemiconductor surface 30 is applied against the polishing pad 26 (block106). Alternatively, those skilled in the art will appreciate that thepolishing pad 26 can be applied against the semiconductor surface 30.When the semiconductor surface 30 is first applied to the polishing pad26, the fluid 48 is preferably in a liquid state and the semiconductorsurface 30 is preferably centered with the polishing pad 26. Also, asdescribed above, an angle θ2 of at least one of the wafer spindle 38 andthe polishing spindle 34 is adjustable.

[0021] Since the fluid 48 is in a liquid or viscous state and at leastone of the spindles 34, 38 is adjustable, the axis of rotation of thepolishing spindle 34 and the wafer spindle 38 automatically becomealigned as the semiconductor surface 30 and the polishing pad 26 engageeach other (block 102). The automatic alignment is accomplished, forexample, by matching the wafer head 36 and the polishing head 32, whereone of the heads 32, 36 is fixed and the other is adjustable. When thetwo heads 32, 36 come into contact, force from the fixed head positionsthe adjustable head. In alternate embodiments, the alignment of thepolishing spindle 34 and the wafer spindle 38 can be accomplished byline-of-sight and manual adjustment or with a laser sight and automaticor manual adjustment.

[0022] When the semiconductor surface 30 and the polishing pad 26 align,the electric coil 50 is energized to create a magnetic field which turnsthe fluid 48 from a liquid to a solid state and thereby maintains thecurrent angle of the axis of rotation of the spindles 34, 38 (block104). Thereafter, the polishing pad 26 is used to polish thesemiconductor wafer surface 30 (block 106). After completion of thepolishing process, the electric coil 50 is shut off to allow the fluid48 to return to the liquid state so that realignment can occur for anext process to run.

[0023] In another embodiment, the electric coil 50 is alternatelyenergized and de-energized to apply and disengage the magnetic field tothe fluid 48. By alternately applying and disengaging or reducing themagnetic field, the fluid is alternately solidified and liquefied, whichallows for the continual adjustment and setting of the angle of the axisof rotation. Continual adjustment of the angle of the axis of rotationcan reduce system vibration, and, as the polishing pad 26 moves acrossthe semiconductor wafer 24 and to the edge of the semiconductor wafer,the polishing pad is held rigid without the edge effect problem of manyconventional systems, such as the gimbal system.

[0024] In yet another embodiment, the electric coil 50 is energized atthe beginning of a series of runs to create a magnetic field which turnsthe fluid 48 from a liquid to a solid state and thereby maintains thecurrent angle of the axis of rotation of the spindles 34, 38 (block104). Thereafter, the polishing pad 26 is used to polish thesemiconductor wafer surface 30 (block 106). After completion of thepolishing process, the electric coil 50 is maintained in an active statesuch that the initial alignment is maintained until the series of runsis complete after which the he coil shut off to allow the fluid 48 toreturn to the liquid state so that realignment can occur for the nextseries of process runs.

[0025] Although the preferred embodiments have been described inreference to a circular polishing application, it can be readily adoptedto other applications that utilize axial alignment to a fixed surface.For example, the preferred embodiments could be used with lathes, millsand lapping tools. Further embodiments could be used in semiconductorprocessing etch or deposition chambers where a rotating wafer chuck(susceptor) must be aligned to first and second facing electrode toachieve parallelism of the chuck and electrode.

[0026] In the etch or deposition chambers, a first electrode surface anda second electrode surface are provided to the system. The semiconductorwafer 24 is provided on one of the first and second electrode surfaces.An axis of the first electrode surface and the second electrode surfaceare aligned to each other. To accommodate alignment, at least one of thefirst and second electrode surfaces includes an adjustable axisperpendicular to the surface of the adjustable axis. After alignment ofthe first and second electrode surfaces, axial alignment of theadjustable surface is set to maintain a position of the adjustable axis.Thereafter, the semiconductor wafer 24 is etched or film is deposited onthe semiconductor wafer 24.

[0027] Although the descriptions teach alignment of rotating bodies,this invention can be applied to any non-rotating system of bodies whichrequire parallelism and axial alignment. It is to be understood thatchanges and modifications to the embodiments described above will beapparent to those skilled in the art, and are contemplated. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

We claim:
 1. A method for aligning a first rotating surface and a secondrotating surface, the method comprising: providing a first rotatingsurface; providing a second rotating surface, wherein at least one of anaxis of rotation of the first and second rotating surfaces isadjustable; and setting the axis of rotation of the adjustable axis witha material that operates to allow adjustment of an angle of the axis ofrotation and then maintain the angle after the axis of rotation of thefirst and second surfaces are aligned to each other.
 2. The methodaccording to claim 1 further comprising alternately adjusting andsetting the axis of rotation of the first surface to the second surface.3. The method according to claim 1 wherein the material comprises one ofthe group of a fluid, a powder and a gas.
 4. The method according toclaim 3 wherein the fluid comprises a magneto-rheological fluid.
 5. Themethod according to claim 4 wherein the act of setting the axis ofrotation comprises applying a magnetic field proximate to themagneto-rheological fluid.
 6. The method according to claim 5 whereinapplication of the magnetic field comprises energizing an electric coil.7. The method according to claim 5 wherein the magneto-rheological fluidbecomes solid after the magnetic field is applied.
 8. The methodaccording to claim 5 wherein the magneto-rheological fluid becomeshighly viscous after the magnetic field is applied.
 9. A method forpolishing a semiconductor wafer, the method comprising: providing apolishing pad including a first surface; providing a semiconductor waferincluding a second surface; aligning an axis of rotation of the firstsurface and the second surface to each other, wherein at least one ofthe first and second surfaces includes an adjustable axis of rotation;setting the axis of rotation of the adjustable surface to maintain aposition of the adjustable axis after the first and second surfaces arealigned; and polishing the semiconductor wafer with the polishing pad.10. The method according to claim 9 further comprising using a fluid toset the axis of rotation of the adjustable axis.
 11. The methodaccording to claim 10 wherein the fluid comprises a magneto-rheologicalfluid.
 12. The method according to claim 11 wherein the act of settingthe axis of rotation comprises applying a magnetic field proximate tothe magneto-rheological fluid.
 13. The method according to claim 12wherein application of the magnetic field comprises energizing anelectric coil.
 14. The method according to claim 12 wherein themagneto-rheological fluid becomes solid after the magnetic field isapplied.
 15. The method according to claim 12 wherein themagneto-rheological fluid becomes highly viscous after the magneticfield is applied.
 16. The method according to claim 12 furthercomprising alternately applying and disengaging the magnetic field tocontinually adjust and set the axis of rotation.
 17. The methodaccording to claim 16 wherein application of the magnetic fieldcomprises applying the magnetic field at different intensities.
 18. Anapparatus for polishing a semiconductor wafer, comprising: a polishingpad including a first surface; a semiconductor wafer including a secondsurface opposing the first surface of the polishing pad, wherein atleast one of an axis of rotation of the first surface and the secondsurface is adjustable; and a fluid around the axis of rotation of theadjustable surface operable to maintain a position of the axis ofrotation after the axis of rotation of the first and second surfaces arealigned, wherein the semiconductor wafer is polished with the polishingpad after the axis of rotation are aligned.
 19. The apparatus accordingto claim 18 wherein the fluid comprises a magneto-rheological fluid. 20.The apparatus according to claim 19 wherein a magnetic field is appliedproximate to the magneto-rheological fluid to set or solidify themagneto-rheological fluid.
 21. The apparatus according to claim 20further including an electrical coil located proximate to themagneto-rheological fluid, wherein application of the magnetic fieldcomprises energizing the electric coil.
 22. The apparatus according toclaim 20 wherein the magneto-rheological fluid becomes solid after themagnetic field is applied.
 23. The apparatus according to claim 20wherein the magneto-rheological fluid becomes highly viscous after themagnetic field is applied.
 24. The apparatus according to claim 20wherein the magnetic field is alternately applied and disengaged tocontinually adjust and set the axis of rotation of the first surface tothe second surface.
 25. The apparatus according to claim 18 furtherincluding a first spindle shaft attached with the polishing pad and asecond spindle shaft attached with the semiconductor wafer to spin thepolishing pad and semiconductor wafer.
 26. The apparatus according toclaim 25 wherein at least one of the first spindle shaft and the secondspindle shaft are enclosed within the fluid.
 27. The apparatus accordingto claim 25 further including at least one fin attached with at leastone of the first spindle shaft and the second spindle shaft wherein theat least one fin is surrounded by the fluid.
 28. A method for etching asemiconductor wafer, the method comprising: providing a first electrodesurface; providing a second electrode surface; providing a semiconductorwafer on one of the first and second electrode surfaces; aligning anaxis of first electrode surface and the second electrode surface to eachother, wherein at least one of the first and second electrode surfacesincludes an adjustable axis perpendicular to the surface of theadjustable axis; setting axial alignment of the adjustable surface tomaintain a position of the adjustable axis after the first and secondelectrode surfaces are aligned; and etching the semiconductor wafer. 29.The method according to claim 28 further comprising using a fluid to setthe axis of rotation of the adjustable axis.
 30. The method according toclaim 29 wherein the fluid comprises a magneto-rheological fluid. 31.The method according to claim 30 wherein the act of setting the axis ofrotation comprises applying a magnetic field proximate to themagneto-rheological fluid.
 32. The method according to claim 31 whereinapplication of the magnetic field comprises energizing an electric coil.33. The method according to claim 31 wherein the magneto-rheologicalfluid becomes solid after the magnetic field is applied.
 34. The methodaccording to claim 31 wherein the magneto-rheological fluid becomesviscous after the magnetic field is applied.
 35. The method according toclaim 31 further comprising alternately applying and disengaging themagnetic field to continually adjust and set the axis of rotation. 36.The method according to claim 35 wherein application of the magneticfield comprises applying the magnetic field at different intensities.37. A method for depositing a uniform film on a semiconductor wafer, themethod comprising: providing a first electrode surface; providing asecond electrode surface; providing a semiconductor wafer on one of thefirst and second electrode surfaces; aligning an axis of the firstelectrode surface and the second surface to each other, wherein at leastone of the first and second electrode surfaces includes an adjustableaxis of perpendicular to the surface of the adjustable axis; setting theaxial alignment of the adjustable surface to maintain a position of theadjustable axis after the first and second electrode surfaces arealigned; and depositing a film on the semiconductor wafer.
 38. Themethod according to claim 37 further comprising using a fluid to set theaxis of rotation of the adjustable axis.
 39. The method according toclaim 38 wherein the fluid comprises a magneto-rheological fluid. 40.The method according to claim 39 wherein the act of setting the axis ofrotation comprises applying a magnetic field proximate to themagneto-rheological fluid.
 41. The method according to claim 40 whereinapplication of the magnetic field comprises energizing an electric coil.42. The method according to claim 40 wherein the magneto-rheologicalfluid becomes solid after the magnetic field is applied.
 43. The methodaccording to claim 40 wherein the magneto-rheological fluid becomesviscous after the magnetic field is applied.
 44. The method according toclaim 40 further comprising alternately applying and disengaging themagnetic field to continually adjust and set the axis of rotation. 45.The method according to claim 44 wherein application of the magneticfield comprises applying the magnetic field at different intensities.